1. Technical Field
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device having upper and lower electrodes interposing an insulation layer therebetween in which an electric field opening part for passing an electric field is formed in the upper electrode and liquid crystal molecules are driven by applying a voltage between the upper and lower electrodes.
2. Related Art
As a display method for liquid crystal displays, generally, a TN (Twisted Nematic) mode has been widely used. However, there is a limitation on the viewing angle according to the display principle of the TN mode. As a method for solving this problem, a horizontal electric field method in which a pixel electrode and a common electrode are formed on a same substrate as a pair of electrodes for driving liquid crystal molecules, a voltage is applied between the pixel electrode and the common electrode so as to generate an electric field approximately parallel to the substrate, and the liquid crystal molecules are driven within a plane substantially parallel to the substrate surface has been known.
As the horizontal electric field method, an IPS (In Plane Switching) type and an FFS (Fringe Field Switching) type have been known. In the IPS type, a pixel electrode having a comb-teeth shape and a common electrode having a comb-teeth shape are combined to be disposed. The comb-teeth shape is formed in a configuration in which one end portion of an opening part, formed for passing an electric field, in the longitudinal direction is closed, the other end portion of the opening part is opened, and the one end portions of each opening part are connected together in a case where a plurality of the opening parts is disposed.
On the other hand, in the FFS type, an upper electrode and a lower electrode interposing an insulation layer therebetween are formed, one between the upper and lower electrodes is assigned as a common electrode, the other is assigned as a pixel electrode, and an opening part, for example, having a slit shape is formed in the upper electrode. Here, as an opening part used for passing an electric field, a thin and long groove shaped opening part in which both end portions of the opening part in the longitudinal direction are closed is formed. When a plurality of the groove-shaped opening parts is disposed, the opening parts are separately disposed.
The opening part is formed by etching an electrode layer thin film. When the opening part is formed, for example, to have a thin and long groove shape, there are many cases that edge portions of the opening part which are end portions in the longitudinal direction are formed to have a round shape or an arc shape. For example, in the FFS type, since an electric field that comes from the lower electrode, passes through the opening part, and progresses toward the upper electrode flows along the pattern of this opening part, and thus, a horizontal electric field is formed along the pattern of the arc shape. Accordingly, when the initial alignment of the liquid crystal molecules are set to be substantially parallel to a long side of the opening part, for example, by using a rubbing process or the like, the liquid crystal molecules are driven by applying a horizontal electric field, liquid crystal molecules located in a straight line portion of the long side of the opening part rotates in a direction perpendicular to the long side from the initial alignment state. However, liquid crystal molecules located in the edge portions of the opening part rotates in a direction perpendicular to the arc shape from the initial alignment state.
When the liquid crystal molecules rotate along the arc shape of the edge portions from its initial disposition state, there is a case where the rotation direction of the liquid crystal molecules is reversed and there is a case where the rotation direction of the liquid crystal molecules changes depending on locations. This phenomenon in which the rotation direction changes depending on the locations is called disclination. In a boundary in which the rotation direction changes, since the liquid crystal molecules may rotate in an undesired direction or cannot rotated, there is a case where the transmittance is lowered and the boundary is visually recognized. The case is referred to as a rotating tilt line, a rotation tilt defect, or simply disclination.
For example, in JP-A-2005-107535, it is described that, when a configuration in which a black matrix of an upper substrate and an edge portion of a pixel electrode of a lower substrate is overlapped in a predetermined area and a liquid crystal is interposed between the two substrates is used, the twist angle of liquid crystal molecules becomes almost 90° to be aligned in a vertical direction due to the interference of the electric field between the black matrix and the pixel electrode as the location of liquid crystal molecules moves from the end portion of the edge portion of the pixel electrode to a center portion. However, in JP-A-2005-107535, it is indicated that the edge portion has a curve shape due to limitation of the exposure process and thus a trace of rubbing, that is, disclination (a rotating tilt line) occurs in a white gray scale level.
The disclination will be described later in more detail, in comparison with embodiments of the invention.
As described above, when the disclination occurs in a part, transmittance in the part is lowered. Generally, when the disclination occurs, display quality may be judged to be lowered. As described above, the end portion of the opening part of the upper electrode in the longitudinal direction has an arc shape for improving the capacity of a process such as an etching process, and the electric field between the lower and upper electrodes passing through the opening part is formed along the pattern of the arc shape, and whereby the disclination occurs. Accordingly, in order to suppress the occurrence of disclination, it is needed to consider the shape, disposition, and the like of the opening part disposed in the upper electrode in relation with the lower electrode.
In addition, since a wiring for supplying a predetermined electric potential to the upper electrode is disposed on a lower layer side relative to the upper electrode, it is needed to remove the lower electrode, an insulation layer, and the like for connecting the wiring to the upper electrode. Accordingly, while a contact hole for connecting the wiring, which is used for supplying the electric potential, and the upper electrode together is provided, an area around the contact hole has a multi-level structure, and thus it is needed to consider the effect of the level difference when the opening part of the upper electrode is disposed.
As described above, although there are limitations such as disclination, the contact hole, and the level difference around the contact hole on the disposition of the opening part in the upper electrode, the disposition of the opening part has an effect on the display quality of the liquid crystal display.